The present application relates to low-power integrated circuits (and to methods and systems which include low-power integrated circuits), and more particularly to low-power integrated circuits which must be able to interface to multiple external supply voltages (and to methods and systems which include such integrated circuits).
One of the basic requirements of digital integrated circuits is some way to avoid operating at improper power voltages. When the supply voltage is too low, it is possible for some digital circuits to enter an unpredictable state. With programmable, binary logic, such an unpredictable state may result in a device which is locked up and inoperable.
To avoid this, integrated circuits very commonly include a special circuit to detect whether the power supply is in its valid range. Such circuits are often referred to as “power-on reset” or POR circuits, since they typically assert a reset signal as soon as they are powered up. The POR circuit stops asserting the reset signal when it detects that the power supply is in its valid range. The logic in the integrated circuit can then begin to execute its operations reliably, since it is starting from a known initial state. The binary logic is predictable without logic fluctuations stemming from out of range voltage.
The POR function is often implemented by including a bandgap voltage reference on the chip. The bandgap voltage reference outputs a fixed reference voltage as soon as it receives sufficient voltage. This fixed reference voltage is compared with a divided-down fraction of the current power supply voltage to determine whether the supply voltage is in the chip's valid range. Typically, this fraction is determined by a pair of resistors. Since the value of the fixed reference is approximately 1.23V and uncontrollable, the value of the fraction determines the power voltage detected.
In interface circuits, it is similarly desirable not to begin data operations while the supply voltage is still powering up to a valid level in the operating range of the chip. This can result in an unpredictable loss of bits or blocks at the beginning of the transmission, or state errors at the receiver.